Conventional diagnostic use of x-radiation takes the form of either radiography where a shadow image of a patient is produced on a photographic film or fluoroscopy where a visible shadow light image is produced by X-rays impinging on a fluorescent screen.
In a typical X-ray tube, electrons are accelerated as a beam from a tube cathode through an evacuated chamber to a tube anode. When the electrons strike the anode with large kinetic energies they experience a sudden deceleration and x-radiation is produced. The X-ray tube includes a window, transmissive to X-rays, spaced from the anode at an appropriate position so that radiation from the anode passes through the window toward a subject undergoing examination or treatment.
Many tube designs use filaments as a source of electrons to bombard the tube anode. A filament is a coil of wire which is eIeotrically energized so that electrons are thermionically emitted and accelerated toward the anode due to a voltage difference between cathode and anode.
The cathode filament is thermionically energized with a relatively low voltage (on the order 10 volts) and high current A.C. signal. Although the peak-to-peak magnitude of the energization signal is low the reference or average potential of the filament is about -75,000 volts D.C. Stated another way, the voltage on the filament with respect to ground is up to -75,000 volts plus or minus a low level alternating current signal needed to boil off electrons from the filament. At these high voltages, filament inputs must be insulated to prevent arcing. The insulation typically is in the form of high voltage cabling and connectors which are expensive and complex in design.
A trend towards shorter exposure times in radiography has dictated a need for greater intensity of radiation and hence higher electron currents. Attempts to increase the intensity while decreasing the focal spot size can cause overheating of the X-ray tube anode. For this reason, various cooling techniques are known and are incorporated into present day X-ray tubes. Primarily among these techniques are use of rotating anodes which limit heat build up on any one anode spot and oil immersion of the entire tube in a circulating bath which dissipates heat build up. U.S. Pat. Nos. 4,097,759 and 4,097,760 which issued on June 27, 1978 and are assigned to the Picker Corporation. These patents concern the problems of heat dissipation in an X-ray tube anode and are incorporated herein by reference.
As the art of radiography and fluoroscopy has matured, a need for multiple size focal spots has developed so that one X-ray tube can be used for different diagnostic procedures. By way of example, it is desirable to be able to produce a one millimeter long spot for general purpose spot films, a 0.6 millimeter spot for a 105 millimeter camera, a 0.3 millimeter spot for arthrograms, and a 0.15 millimeter spot where magnification is required.
One way to change the size of the focal spot is to use multiple cathode filaments mounted on a single cathode support cup. A dual filament cathode for producing different size focus spots on one X-ray tube anode is shown and discussed in U.S. Pat. No. 4,109,151 to Pleil. Focal spot size can also be controlled to some degree by placing a bias voltage between cathode cup and filament.
Proposals to adjust the current flow with a bias potential in the vicinity of a filament or to increase the number of filaments beyond two have been difficult to accomplish. The reason for this difficulty is the insulating requirements for the high voltage inputs needed for biasing and/or filament energization.
U.S. Pat. No. 4,373,144, entitled "Cathode Arrangement for an X-ray Tube", which was issued Feb. 8, 1983, discusses a procedure for enhancing the flexibility of an X-ray tube in producing multiple size focal spots. The proposal in the '144 patent is to divide a cathode focusing cup into multiple segments which are insulated from each other and then controllably energize those segments, thereby changing the focusing effect the cathode cup has on the size and shape of the focal spot. This proposal may have merit but is apparently accomplished at the expense of added complexity in the routing of signals to the tube anode. There is no indication of how the proposal of the '144 patent is to be implemented. In addition, the '144 patent does not shield the insulating segments against metal deposits. Metal deposits from the filament can short circuit the cup segments resulting in failure to maintain proper voltage levels between segments.